CRITICAL WEIGHTS FOR METAMORPHOSIS IN THE TOBACCO HORNWORM, MANDUCA SEXTA

Fifth-instar larvae of the tobacco hornworm underwent a supernumerary larval molt under several feeding regimens. Supernumerary molting occurred only when the molt was initiated at a live weight less than about 3 g; at higher weights pupation took place. The supernumerary sixth instars were normal in appearance and behavior and went on to form normal pupae only when the fifth instar had begun at weights less than 1 g. Otherwise the supernumerary sixth instars exhibited precocious metamorphosis of their crochets and imaginal discs and failed to undergo normal pupation. Thus, the commitment to pupal development appears to occur in stages. A first stage, initiated at a weight of about 1 g, commits the crochets and imaginal discs to metamorphosis at the next molt. A second stage, entered at a weight of about 3 g, permits the full and complete pupation of the remaining tissues; this transition probably reflects the elimination of juvenile hormone (JH). Starvation apparently alters the normal patterns of ecdyso...